1. Field of the Invention
The present invention relates to a centrifugal air blower, and more particularly to a centrifugal air blower for use in an air-conditioning unit mounted on vehicles.
2. Description of the Related Art
Heretofore, air-conditioning units for use on vehicles incorporate a centrifugal air blower including a fan for drawing in air from within the passenger compartment or from outside the passenger compartment and a motor for rotating the fan (see Japanese laid-open patent publication No. 2003-328994).
FIG. 16 of the accompanying drawings schematically shows in cross section a centrifugal air blower of the related art. As shown in FIG. 16, the centrifugal air blower, generally denoted by 100, comprises a rotational drive source 102, a fan 104 rotated by the rotational drive source 102, and a housing 106 including a holder 108 which holds the rotational drive source 102 in surrounding relation thereto.
The housing 106 has a coolant passage 110 defined therein for passing therethrough cooling air for cooling the rotational drive source 102. The coolant passage 110 has an opening 112 in one end thereof.
When the rotational drive source 102 is energized, it rotates the fan 104 to deliver air into the passenger compartment. If the rotational drive source 102 is continuously energized over a long period of time, then the rotational drive source 102 itself generates heat. To prevent the rotational drive source 102 from generating too much heat, part of the air flow generated by the fan 104 is guided through the opening 112 into the coolant passage 110 as indicated by the arrows and applied to the outer circumferential surface of the rotational drive source 102 to cool the rotational drive source 102.
However, the cooling air (returning air) which has cooled the rotational drive source 102 and the air delivered from the fan 104 tend to interfere with each other on the housing 106, producing a resonant sound. Furthermore, the returning air and the air delivered from the fan 104 interfere with air flowing through a bell mouth of a casing which houses the rotational drive source 102, the fan 104, and the housing 106, also producing a resonant sound.